【作者】 吴厚平；

【导师】 黄伯云； 张立；

【作者基本信息】 中南大学 ， 材料学， 2011， 博士

【摘要】 稀土在硬质合金中定向迁移与在合金烧结体表面的富集现象在国际上尚未见其他类似报道。本课题组的前期研究主要集中在稀土独立掺杂WC-Co合金烧结过程中稀土的表面富集条件、富集形式与富集机理。前期研究结果表明,烧结过程中稳定存在于合金内部的高活性稀土在一定服役条件下可以向合金工具工作表面产生定向迁移,在工作表面原位形成Ln2O2S(Ln:稀土)薄膜,从而明显改善合金使用性能。本研究主要包括以下三个方面：RE(RE:特指以La、Ce为主体成分,含Pr、Nd的高活性混合稀土)在Cr/V-RE联合掺杂WC-Co合金烧结过程中的定向迁移行为；La在Cr/V-La2O3联合掺杂WC-Co合金烧结过程中的定向迁移行为以及WC-Co合金烧结体表面稀土弥散相形成机理及其相成份、相结构影响机理与功能作用。本研究的主要目的是通过对稀土在Cr/V联合掺杂WC-Co合金烧结过程中的定向迁移行为与机理研究,实现对稀土在硬质合金中定向迁移行为的有效控制。因La/Ce原子与W/Co原子的原子半径相差29.36～48.97%,如何激活与控制稀土在硬质合金烧结过程中的定向迁移行为不仅具有重要的科学意义,而且对稀土在硬质合金中作用机理研究、稀土改善硬质合金使用性能机理研究以及(自润滑)功能导向型高性能超细硬质合金的开发均具有重大意义。选用RE-Co预合金粉与高纯纳米La2O3作为稀土掺杂原料；利用扫描电镜、能谱分析仪、X射线衍射仪与光学显微镜,采用合金烧结体表面观察与分析法,结合合金内部组织结构观察与分析、合金热力学分析等手段,研究了稀土在Cr/V联合掺杂WC-Co合金烧结过程中的定向迁移行为与机理。研究结果表明,对Cr/V-RE联合掺杂WC-Co合金,烧结过程中合金中的稀土可以产生定向迁移,在合金中呈弥散分布的稀土相中的部分La、Ce、Pr、Nd原子可以脱离其原始结合状态,通过定向迁移在合金烧结体表面形成新的稀土弥散相。对Cr//V-La2O3联合掺杂WC-Co合金,当合金为WC+β+η三相组织时,烧结过程中合金中的La可以产生定向迁移,在合金烧结体表面形成稀土弥散相。无论是以液态形式迁移还是以固态形式迁移,稀土原子/离子的定向迁移必须在合金内部体系(存在高活性游离C原子)与外部环境(烧结炉内气氛中存在S、O等微量杂质元素)同时满足一定条件下才能发生,因此稀土原子/离子的迁移行为是可控的。同时满足在Co中具有较高溶解度以及高碳碳化物分解为低碳碳化物激活能低的碳化物均具有高活性游离C原子供给能力,因而可成为稀土迁移活性的激活基元。由于WC骨架结构的存在,即使稀土原子以液态形式迁移,也不可避免遇到WC迁移壁垒,因此稀土的原子极化是稀土原子/离子穿越WC晶格从而产生快速迁移的必要条件。存在稀土原子极化的条件下,由于La系收缩效应以及Ce4+、Pr4+离子的存在,当原子尺寸因素成为迁移速度的主要影响因素时,与La比较,Ce、Pr、Nd具有快速迁移优势。WC-Co合金中稀土原子产生极化与定向迁移所需的激活能与掺杂稀土的原始活性有关。当WC-Co合金含碳量正常时,Cr、V高碳碳化物分解为低碳碳化物的驱动力降低,La2O3中La的迁移活性难以得到激发。随烧结炉内气氛中S相对逸度的增加,合金烧结体表面依次可形成LnCoO3、Ln2O2S、Ln2S3等稀土弥散相。因Ce2O2S与Ce2S3的ΛfGVO(单位体积生成自由能变化)明显较相应的La2O2S与La2S3低,在Cr/V-RE联合掺杂WC-Co合金的烧结过程中更有利于Ce2O2S或Ce2S3型晶体结构稀土弥散相的形成。在体系中S相对逸度较高的条件下,S向合金内部的迁移速度增加。与La、Pr、Nd相比,因Ce更易与S形成化合物,随着S原子的向合金内部迁移以及S原子与部分Ce原子的优先结合,Ce较La的定向迁移优势被削减,Ce原子的迁移总量降低。与此同时,迁移通道的开启有利于加快La的定向迁移速度,因此导致了合金烧结体表面高La/Ce比Ln2S3弥散相的形成。合金表面原位形成的稀土弥散相具有较好的耐磨性,与合金基体具有较好的结合强度,能赋予合金自润滑功能与高耐热性能。更多还原

【Abstract】 The directional migration and enrichment phenomena of Ln (Ln: rare earth) on the sinter skin of Ln doped cemented carbide have been discovered for the first time by our research group. Similar report has not appeared worldwide. During the early stage research, we mainly concentrated on the study of the existing form of Ln both in the alloy and on the sinter skin, the conditions and mechanism of the enrichment of Ln on the sinter skin of Ln solo doped WC-Co alloy. It was shown that a Ln2O2S film can be formed in situ on the working faces of the as-sintered cemented carbide tool under some kind of service condition, when Ln can be confined in the alloy during the sintering process. As a result, the performance of the alloy can be effectively improved. This research mainly focus on the following aspects:the directional migration behavior of La, Ce, Pr and Nd in Cr/V-RE jointly doped WC-Co alloy during the sintering process, where RE represents mischmetal containing La, Ce, Pr and Nd with La and Ce as the main composition and is added in the form of RE-Co alloy; the directional migration behavior of La in the Cr/V-La2O3 jointly doped WC-Co alloy during the sintering process;the mechanisms for the formation of the Ln containing dispersed phase(s) and for the variation of the phase composition and structure, as well as the self lubrication function of the Ln containing phases. This research is aimed at the full controlling of the migration behavior of rare earth in the alloy. As is known, there are huge difference in the atomic radius between La/Ce and W/Co,29.36～48.97%. Therefore, how to stimulate and get control over the directional migration behavior is not only of great scientific significance, but also very important in the study of the mechanisms for the effect of Ln on the microstructure and properties, the performance improvement of cemented carbide in service, as well as the development of the self-lubricated function oriented ultrafine cemented carbide with high performance.Mischmetal and cobalt pre-alloyed powder with high activity, and high pure nano sized La2O3 with low activity were selected as Ln doping raw materials. SEM, EDS, X-ray diffractometer and optical microscope were used as the observation measures. The sinter kin observation and analysis method, coped with the inner microstructure observation and analysis, as well as the thermodynamic analysis were used in the study of the Ln atoms/ions directional migration behavior and the related mechanism.It is shown that during the sintering process of Cr/V-RE jointly doped WC-Co alloy, La, Ce, Pr and Nd can migrate directionally, the Ln containing dispersed phase (M) in the alloy with WC+β+M microstructure is thermodynamically unstable and part of the La, Ce, Pr and Nd atoms can reunion on the sinter skin, forming in situ a new Ln containing dispersed phase. During the sintering process of Cr/V-La2O3 jointly doped WC-Co alloy with WC+β+ηmicrostructure, La can migrate directionally, forming in site La containing dispersed phase. Whether the Ln atoms/ions is in the form of liquid or solid, the directional migration of Ln atoms/ions must meet both the internal and external environment conditions, i.e. there exist free carbon atoms with high activity in the alloy and there exist S and O impurity atoms in the sintering atmosphere. Therefore, the directional migration behavior of Ln atoms/ions is controllable. Carbide with high solubility in cobalt and with lower stimulation energy for the decomposition of high carbon type into low carbon type can be the stimulation primitive unit of the migration activity of Ln atoms/ions for its offering ability of free carbon atoms with high activity. Because of the existence of WC skeleton structure and the resulted migration barrier, even if the migration of Ln atoms is in the state of liquid, the polarization of Ln atoms is the requisite for the migration of Ln atoms through WC lattice and the resulted fast migration. Under the situation of the polarization, compared with La, Ce and Pr and Nd have the advantage of fast migration when the atomic size factor becomes the controlling one for the migration, because of the lanthanide contraction and the existence of the Ce4+ and Pr4+ irons. The activation energies of the Ln atoms polarization and migration are related to their original activity. For La2O3 doped WC-Co alloy with normal carbon content and the resulted WC+βtwo phase structure, the driving force for the decomposition of Cr3C2 or/and VC into the lower carbon type is decreased;as a result, the migration activity of La ions cannot be activated. With the increasement of the fugacity of S in the sintering atmosphere, LnCoO3, Ln2O2S and Ln2S3 dispersed phase(s) can be formed in turn on the sinter skin of Ln-Cr/V unitedly doped WC-Co alloy. Compared with the values ofΔfGv0 of La2O2S and La2S3, the respective ones of Ce2O2S and Ce2S3 are lower; therefore, the crystal structure of Ln2O2S and Ln2S3 on the sinter skin of Cr/V-RE jointly doped WC-Co alloy are Ce2O2S and Ce2S3 type, respectively. In the situation of relatively higher fugacity of S in the sintering atmosphere, the migration speed of S atoms into the alloy can be accelerated. As a result, parts of the Ce atoms in the alloy combine preferentially with the S atoms for the lower Gibbs free energy compared with the ones of La, Pr and Nd. The advantage of the migration of Ce atoms is thereby cut down and the migration total amount of Ce atoms is deduced. In the mean while, the opening of the migration channel facilitates the fast migration of La atoms; therefore, the Ln2S3 dispersed phase with higher La/Ce ratio than the one in RE-Co powder can be formed. There is good combining strength between the Ln containing dispersed phase and the WC+βmatrix and good wear resistance of Ln containing dispersed phase. The formation of Ln containing dispersed phase on the skin, no mater it is in the form of LnCoO3, Ln2O2S or Ln2S3 can endow the alloy with self lubrication and high heat resistance functions.更多还原